(1) Field of the Invention
The present invention relates to light sources, and relates particularly, but not exclusively, to light sources comprising solid state laser diodes.
(2) Description of the Prior Art
Laser diodes are compact, robust, efficient and relatively inexpensive sources of laser light. It has therefore been proposed to use laser diodes as light energy sources in many applications in place of previously used gas lasers or solid state lasers such as Nd:YAG which are large and not easily portable. Such applications include body implantable angioplasty probes, ophthalmic treatments, contact laser surgery, etc.
However, the power output available from a single laser diode is limited to a few watts. Furthermore, each diode emits from an elongated high aspect ratio "stripe" into a relatively large cone angle. The cone angle, i.e. the numerical aperture of the emitted beam, is smaller in the direction parallel to the long axis of the laser stripe (hereinafter called the x-axis) than in the direction perpendicular to the long axis of the stripe (y-axis).
In view of the power limitation of a single laser diode, for many applications it is necessary to combine the outputs from a number of laser diodes. In any such system, the light from a number of laser diodes must be efficiently transmitted to a target area, which typically may have an aspect ratio which is lower than that of the laser stripe and which also may have a certain maximum acceptance cone or solid angle for efficient transmission of light to the target. For example, in applications such as angioplasty devices where light energy is transmitted through an optical fibre, the target area, i.e. the end of the fibre, is round, and the acceptance solid angle is the same in both axes and corresponds to the maximum numerical aperture of an incident beam which can be efficiently coupled into the fibre. A further example is treatment of the retina in the eye, where the light energy must be focussed onto a predetermined target area on the retina via the iris which imposes a maximum acceptance solid angle.
Problems arise in practice in seeking to combine light from a plurality of laser diode sources such that the required power may be delivered to a predetermined target area within the constraints imposed by the efficient acceptance solid angle associated with the target or indeed with the delivery optics of the source itself.
There has been a number of approaches to overcome the problem. It has been proposed to combine the beams from two laser diodes using a polarising beam combiner and to focus the combined beam onto the end of an optical fibre. It has also been proposed to minimise the diameter of the fibre in such an arrangement by anamorphically demagnifing the image of the laser stripe in its long dimension up to a point where the numerical apertures of the combined laser beam in both dimensions match the acceptance numerical aperture of the fibre. However, for either of these proposals the maximum power for each fibre is equivalent to that from only two laser diodes, and to achieve more power it is necessary to bundle a plurality of such fibres together. In certain applications this may not be an efficient solution in view of image to target size mis-match and in view of the complex fibre optics required. The bundling of a number of fibres together in this way also results in a loss of brightness and at high power can be prone to thermally induced damage. It has also been proposed to combine beams from a number of lasers of different wavelengths using wavelength selective mirrors. However, this is unsatisfactory in practice, in view in particular of the extreme temperature sensitivity of the wavelength of laser diodes. A further approach has been to form the end of optical fibres into an oblong shape to match more closely the laser diode stripe, but such fibres are expensive to produce and the coupling of light energy into each fibre is not particularly efficient.
A still further attempt at providing a more powerful laser diode light source may be found in U.S. Pat. No. 4,905,690 in which light beams from a number of laser diodes are arranged to have their axes parallel, and to be focussed on to a target area by a single focusing lens having a diameter sufficient to encompass the multiple beams. An example of this technique applied to a simple imaging system is shown in FIG. 1 from both a top and side view. The system comprises a collimating lens 1 and a focusing lens 2 of focal lengths f1 and f2, respectively, and is frequently used to couple a single laser diode to an optical fibre by focusing an image 3 of the laser emission stripe 4 onto the end of the fibre. The stripe 4 has a long dimension a in the x-axis and a short dimension b in the y-axis and is magnified by the lenses 1, 2 in both axes by a factor f2/f1. The diameter of the collimated beam 5a of a laser diode in the y-axis is typically three times that in the x-axis due to the difference in the numerical apertures of the emitted beam in the two axes. Therefore, and in accordance with U.S. Pat. No. 4,905,960, two further parallel beams 5b, 5c (stacked in the x-axis direction) can be placed substantially within the aperture of the focusing lens 2. This increases the power and intensity of the image by a factor of 3. However, if more power is required and the arrangement is therefore extended to include more than three beams, problems arise because it becomes necessary to increase the diameter of the focusing lens in order to encompass, the extra beams (this is illustrated for five beams in FIG. 2), and if the same numerical aperture of the focussed beam is to be maintained so that it stays less than or equal to the numerical aperture of the fibre to avoid power loss, then the focal length of the focusing lens must also be increased in direct proportion to the increased diameter. This increased focal length (by a factor of 5/3 in the case of FIG. 2) increases the magnification of the stripe images by the same factor so that although the power has been increased, the size of the image has also been increased in the same proportion. This will result in power loss if the image is magnified to be larger than the target area, i.e. the fibre end diameter.
In "Scalable, end-pumped, diode-laser-pumped laser" (Optics Letters, Vol. 14, No. 19, Oct. 1, 1989), and in "Pump Source Requirements for End-Pump Lasers" (IEEE Journal of Quantum Electronics, Vol. 26, No. 2, February 1990), Fan et al teach a laser diode system for pumping Nd:YAG lasers (see FIG. 3) in which a number of parallel laser diode beams 6a, 6b, 6c (stacked in the y-axis as opposed to the x-axis) are focussed into a gain medium of the Nd:YAG laser by a single pair of crossed cylindrical lenses (6, 7), each focusing independently in planes orthogonal to one another (xz and yz planes respectively). This allows the focal lengths (f3, f4) of the focusing lenses to be optimised independently in the two axes. Therefore, the more critical long x-dimension of the image 3 may be optimised, within the constraints of the source and collimating lens properties and the numerical aperture of the focused beam, by selecting the focal length f3 of the cylindrical lens 6 focusing in that axis accordingly, e.g. so that size of the image 3 does not exceed the target area in the x-direction. In the less critical short y-dimension of the image 3, the aperture size is allowed to increase to accommodate a number of beams. This again necessitates an increase in the focal length f4 of the lens 7 in order to maintain the numerical aperture of the focused beam less than or equal to that of the fibre to avoid power loss. This increase in focal length again produces a commensurate increase in the y-dimenson of each image and reduction in the y-axis numerical apertures of each beam to allow more beams to fit into the acceptance cone angle of the target. Thus, and unlike in the U.S. Pat. No. 4,905,690 system, if this system is extended to increase the number of beams focused on to the target area the numerical aperture of the focused beam may be maintained without needing to increase the magnification of the image in the long x-axis but only in the less critical short y-axis. Therefore, this system allows higher powers to be achieved into a given fibre diameter than does U.S. Pat. No. 4,905,690.
This system does however have a number of disadvantages. First, for focused beams of reasonable numerical apertures, complex multi-element or aspheric cylindrical lenses will be required to reduce optical aberrations to an acceptable level and to compensate for aberrations in the y-axis imaging caused by the x-axis cylinder. Moreover, while multi-element or aspheric symmetrical lenses are standard practice in the optics industry, the manufacture of multi-element or aspheric cylindrical lenses is non-trivial and almost unknown in the optics industry.
Second, in order to combine a reasonable number of beams, the aperture size of the y-axis focusing cylinder must become very large. Therefore, in order to maintain the same numerical aperture for the focused beams in the y-axis, the focal length of the y-axis focusing cylinder must also increase in proportion to the number of beams, which means that the distance between the diodes and the x-axis focusing cylinder becomes correspondingly large. This causes the problem of significant divergence of the beams over such a distance in the x direction. As an example, in their publications Fan et al combine 3 diodes using a 150 mm focal length 25 mm aperture cylindrical lens for focusing in the y-axis, if this were increased to 16 beams under similar constraints, the aperture of the lens would increase to 155 mm and its focal length to just under one meter (930 mm). In the orthogonal (x-axis) plane the original 20 mm focal length lens would still need to be used but because this lens is now more than 910 mm from the diodes, the 25 mrad divergence of the collimated beam in this axis (arising from the source size in this axis) will result in an increase in the beam dimension at the lens aperture from approximately 4 mm in the case they illustrated in their publication to approximately 30 mm, i.e. the numerical aperture of focused beam in this axis has increased from 0.1 NA to 0.6 NA. For their demonstration Fan et al used sources with a 100 .mu.m strip dimension in the x-axis and a 4 mm focal length collimating lens and this is the basis for the above divergence calculation. For higher power sources, 200 or 500 .mu.m sources are typically required and the divergence problem would be increased by a factor of 2 or 5 respectively.